It is known for subscriber units (also interchangeably referred to herein as radios) to operate in “Direct Mode” (also referred to herein as “TDMA Direct Mode”). Direct Mode is defined as a mode of operation for two or more subscriber units, wherein the subscriber units communicate with each other without the need for any infrastructure equipment, such as a base station or repeater. This is in contrast to “repeater mode” defined as a mode of operation for two or more subscriber units, wherein the subscriber units communicate with each other using one or more pieces of infrastructure equipment. For instance, the European Telecommunications Standard Institute (ETSI) has published ETSI TS (Technical Specification) 102 361-1 (wherein the current version v1.4.5 dated December 2007 and all future versions are referred to herein as the ETSI-DMR standard), which describes a repeater mode and direct mode of operation for subscriber units.
The TDMA channel access method allows multiple stations (either mobile subscriber units and/or fixed end stations) to share the same transmission medium by dividing the transmission medium into time slots (or simply slots), wherein a time slot is defined as an elemental timing of a physical channel and is defined in 30 ms increments in the ETSI-DMR standard. A given TDMA system is characterized by the number of time slots supported in the system. For instance, a two-slot (or 2-slot) TDMA system (as described in the ETSI-DMR standard as 2:1 mode) supports two time slots for each physical channel. A four-slot TDMA system supports four time slots for each physical channel, and so on.
Ideally, each time slot supports a different traffic channel and, thereby, a different call. However, to avoid interference between transmitting subscriber units using the same physical channel (i.e., the same radio frequency channel) but different time slots, each subscriber unit should be aware of a common “channel timing” for the physical channel. As used herein, channel timing is defined as a timing reference that defines boundaries for one or more of the time slots that make up the physical channel. In other words, the channel timing defines a common time slot boundary reference for transmissions on the physical channel. Accordingly, the phrases “channel timing” and “time slot boundary reference” are used herein interchangeably.
The ETSI-DMR standard currently describes a means for an infrastructure device to provide common channel timing for subscriber units operating in repeater mode. Therefore, when the subscriber units are operating in repeater mode, two different non-interfering calls can be supported on the two time slots of a two-slot TDMA system. However, the ETSI-DMR standard does not presently describe a means for providing common channel timing to subscriber units operating in direct mode.
Therefore, what is needed is a technique for diffusing channel timing among subscriber units operating in TDMA direct mode.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative other elements to help improve understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required.
Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood elements that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.